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Nitration

Nitration is a general class of chemical process for the introduction of a nitro group into an organic chemical compound. More loosely the term also is applied incorrectly to the different process of forming nitrate esters between alcohols and nitric acid, as occurs in the synthesis of nitroglycerin. The difference between the resulting structure of nitro compounds and nitrates is that the nitrogen atom in nitro compounds is directly bonded to a non-oxygen atom, typically carbon or another nitrogen atom, whereas in nitrate esters, also called organic nitrates, the nitrogen is bonded to an oxygen atom that in turn usually is bonded to a carbon atom (nitrito group).

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Typical nitration syntheses apply so-called "mixed acid", a mixture of concentrated nitric acid and sulfuric acids.[2] This mixture produces the nitronium ion (NO2+), which is the active species in aromatic nitration. This active ingredient, which can be isolated in the case of nitronium tetrafluoroborate,[3] also effects nitration without the need for the mixed acid. In mixed-acid syntheses sulfuric acid is not consumed and hence acts as a catalyst as well as an absorbent for water. In the case of nitration of benzene, the reaction is conducted at a warm temperature, not exceeding 50 °C. [4] The process is one example of electrophilic aromatic substitution, which involves the attack by the electron-rich benzene ring:

Alternative mechanisms have also been proposed, including one involving single electron transfer (SET).[5][6]

Selectivity can be a challenge in nitrations because as a rule more than one compound may result but only one is desired, so alternative products act as contaminants or are simply wasted. Accordingly, it is desirable to design syntheses with suitable selectivity; for example, by controlling the reaction conditions, fluorenone can be selectively trinitrated[7] or tetranitrated.[8]

The direct nitration of aniline with nitric acid and sulfuric acid, according to one source [9] results in a 50/50 mixture of para and meta nitroaniline. In this reaction the fast-reacting and activating aniline (ArNH2) exists in equilibrium with the more abundant but less reactive (deactivated) anilinium ion (ArNH3+), which may explain this reaction product distribution. According to another source [10] a more controlled nitration of aniline starts with the formation of acetanilide by reaction with acetic anhydride followed by the actual nitration. Because the amide is a regular activating group the products formed are the para and ortho isomers. Heating the reaction mixture is sufficient to hydrolyze the amide back to the nitrated aniline.